I tried to test chainlink vrf at hardhat test with vrfcoordinatormock. test is work fine but random words and request id are always 0. is It correct that returning 0 from MockVRF?

    const deployContract = async(vrfCoordinatorContract : "MockVRFCoordinator" | "MockVRFCoordinatorUnfulfillable" = "MockVRFCoordinator") => {
        const Consumer = await ethers.getContractFactory("VRFv2Consumer");
        vrfCoordFactory = await ethers.getContractFactory(vrfCoordinatorContract);
        vrfCoordinatorV2Mock = await vrfCoordFactory.deploy();
        return await Consumer.deploy(MOCK_SUBSCRIPTION_ID,keyHash,vrfCoordinatorV2Mock.address);


    before(async () => {
        const [owner] = await ethers.getSigners();
        owner_wallet = owner.address;
        const LotterPoly = await ethers.getContractFactory("LotterPoly");
    it("Should successfully request a random number", async() => {
        vrfv2Consumer = await deployContract();

        let tx = await vrfv2Consumer.requestRandomness();
        await tx.wait();
        await new Promise(resolve => setTimeout(resolve, 30000));
        const requestId = await vrfv2Consumer.s_requestId();
        const randomWord = await vrfv2Consumer.s_randomWords(0);


1 Answer 1


Random words and request id should not be 0. If you check VRF mock contract in VRFCoordinatorV2Mock.sol, you will find that the logic to generate random words is defined in the function fulfillRandomWordsWithOverride, and the code shown below:

if (_words.length == 0) {
  _words = new uint256[](req.numWords);
  for (uint256 i = 0; i < req.numWords; i++) {
    _words[i] = uint256(keccak256(abi.encode(_requestId, i)));
} else if (_words.length != req.numWords) {
  revert InvalidRandomWords();

The snippet of codes shows that random words are generated by encoding the _requestId and i in the for loop, so the random number should not be 0.

In your test, actually, you do not verify any result at all. If you want to verify the random words is received and greater than 0, maybe you can get the random words first and use assertion to verify it.


Please check the unit test in the repository Chainlink hardhat starter kit.

  • I checked Chainlink hardhat starter kit and executed test at starter kit. but starter kit also can't make random word too.
    – Dean97K
    Commented Aug 4, 2022 at 8:07
  • Hi @Dean97K, can you describe the steps so that I can reproduce your issue.
    – Frank Kong
    Commented Aug 5, 2022 at 2:14
  • let me just explain about hardhat-starter-kit, I just git clone from link and I just exectuted npm install , npm test (hardhat test test/unit/*_test.js --network hardhat) then request random number test failed. I changed nothing though.
    – Dean97K
    Commented Aug 6, 2022 at 8:07
  • I did exactly the same thing as you and randomConsumer test passed. Can you rephrase your question to describe how you test and what error you got in details?
    – Frank Kong
    Commented Aug 7, 2022 at 3:05
  • Hi, I did make new project several times. It sovled. I don't know why but it works. I think library crash. thanks for your help!
    – Dean97K
    Commented Aug 9, 2022 at 8:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.